Described below is a method of operation in a dual carrier communication system, in particular for global system for mobile communications (GSM) systems, such as general packet radio service (GPRS) or voice group call systems.
GSM Edge Radio Access Network (GERAN) currently operates on one carrier at a time, but to increase capacity on the downlink, dual carrier operation is being introduced. Typically, general packet radio service (GPRS) mobility, without the use of packet switched (PS) handover, is handled by a cell reselection procedure. In the cell reselection procedure, a mobile station (MS) detects that a neighboring cell has a stronger signal than that of its current cell and decides, either autonomously, or via network control, to reselect to the new cell. The MS drops its GPRS connection in the old cell, reads system information in the new cell, sends access bursts to request resources in the new cell, and finally performs a cell update to indicate to a serving GPRS support node (SGSN) in the network that the MS has moved to the new cell. Upon receiving the cell update, the SGSN redirects the downlink data flow to the MS.
During this time no downlink data is being received by the MS and only when the cell update takes place can uplink data be sent to the core network. It also assumes that the transfer will be successful, by dropping its existing connection at the start, although in some cases, the new cell may not have the resources available to allocate to the MS, requiring it to try to reconnect elsewhere. GPRS service interruption of up to 5 seconds may be experienced.
For voice group calls using voice group call service (VGCS) or voice broadcast service (VBS), a MS in group receive mode reads system information in a neighboring cell in order to determine the position of a notification channel (NCH) and obtain some cell specific parameters such as the cell's identity which is needed to derive the ciphering keys. After cell reselection, the MS reads the NCH, if present, to find a location and description of the group, or broadcast channel for the particular group, or broadcast call. During this time the MS may not be able to listen to the downlink of the group, or broadcast channel in the new cell. Thus, there may be a noticeable interruption to the reception of speech on cell change. Additionally, the MS may miss the downlink signalling messages that are sent on the group, or broadcast channel. These signalling messages can indicate the current status of the call (free or busy in VGCS only), the identity of the current talker (in VGCS only), new notifications (in both VGCS and VBS), or incoming short message service (SMS), either to the group or to the individual MS (applicable for both VGCS and VBS). Uplink BUSY messages are typically transmitted every 5 seconds, so for VGCS, the MS may have to wait some time to determine the current status of the uplink.
One proposal to speed up cell reselection for GPRS services has been to introduce a process of network assisted cell reselection, whereby the network can send the system information of the new cell to the MS whilst it is still in the old cell. This requires a radio access network (RAN) information management (RIM) implementation to automatically transfer system information from neighbor cells, or an operations and maintenance (O&M) application to configure neighbor cell information. Even with a network assisted cell change (NACC), there is still some extra service interruption time due to the MS accessing the new cell, being allocated resources, performing a cell update and waiting for downlink data to be received.
Another option is to use PS handover procedures as defined for Release-6 of 3GPP GERAN standards, whereby resources are reserved in the target cell and core network signalling is used to prepare the target before the MS is commanded to handover to the target cell. This has the best performance in terms of service interruption, but requires complex core network signalling and is wasteful of radio resources in the target cell, which are reserved, but not used for a period of time. PS handover is aimed at being used for real-time PS services where the revenue is worth the cost of the procedure.
For VGCS or VBS, there is no option but to simply accept a service interruption time.